Lecture 24: Beyond the Octet Reading: Zumdahl Outline –Sub-octet systems –Valence-shell expansion –Radicals: odd-electron systems

Beyond the Octet Rule There are numerous exceptions to the octet rule. We’ll deal with three types of exceptions here: 1. Sub-octet systems (less than 8 electrons) 2. Valence shell expansion (more than 8 electrons) 3. Radicals ( an odd number of electrons)

Sub-Valent Systems Some atoms (for example, Be, B, and Al) undergo bonding, but will form stable molecules that do not fulfill the octet rule. Experiments demonstrate that the B-F bond strength is consistent with single bonds only.

Sub-valent molecules will react with other molecules such that the octet rule is satisfied. The octet rule is satisfied by the reacting partner providing an electron pair. A bond in which both electrons come from one of the atoms is called a coordinate covalent bond.

Valence Shell Expansion For third-row elements (Period 3) and beyond, the available nearby d-orbitals allows for the participation of these orbitals in bonding. When this occurs, more than 8 electrons can surround a third-row element. Example: ClF 3 (a 28 e - system) F obey octet rule Cl has 10e -

Typical atoms that demonstrate valence-shell expansion are P, S, and larger halogens (Cl, Br, and I). Example: PCl 5 (40 electrons)

Lewis-dot structure for valence shell expansion: Assign all electrons to bonds and electrons so as to give each atom an octet. For halogen complexes (like PCl 5 ), expand the octet of period-3 elements using single bonds until halogens are bonded. Finally, assign any remaining electrons to the period-3 element. Example: I e -

Bonding in molecules containing nobel-gas atoms involves valence shell expansion A famous example: XeO 3 26 e

Radicals: Odd-Electron Systems Finally, one can encounter odd electron systems where full electron pairs will not exist. Example: Chlorine Dioxide. Unpaired electron 19 e -

Radicals: Odd-Electron Systems Strategy: Generally, attempt to put the odd electron on the central atom. However, the LDS rules were not intended to be applied to odd-electron systems. Example: NO e - N is sub-octet; therefore, predict that NO 2 is highly reactive.

Lewis Dot Rules Summary Remember the following: *(C, N, O, and F) almost always obey the octet rule. *(B, Be, and Al) are often sub-octet *Second row (Period 2) elements never exceed the octet rule *Third Row (Period 3) elements and beyond can use valence shell expansion to exceed the octet rule. In the end, you have to practice…..a lot!